power: fg-alg: Fix TTF calculation to converge to correct TTF faster

When charger is plugged in, initial TTF value can be incorrect if
Ibatt value is not settled. Fix this by clearing ibatt samples buffer
once the current Ibatt reading differs by 500 mA with the Ibatt_avg,
to converge to accurate TTF value.

Change-Id: Id3c8d227d575200d25ffbb5c314877ce4fe99d73
Signed-off-by: Sahil Chandna <chandna@codeaurora.org>